1. Field of the Invention
The present invention relates to an ultrasonic Doppler diagnosis apparatus using a color mapping technique for two-dimensionally displaying an image of blood flowing in a heart or vessels.
2. Description of the Related Art
A conventional ultrasonic Doppler diagnosis apparatus is developed based on the principle that an amount of deviation between a transmission frequency and a reception frequency, that is, a deviation frequency between them depends upon a speed component of a moving object (corpuscles) in the direction of an ultrasonic beam. Flowing blood is imaged in blue/red and others in accordance with the positive/negative of the deviation frequency and its brightness is varied with the level of the deviation frequency.
Assuming that the transmission frequency is f0, the reception frequency is f1, the deviation frequency is fd, the speed of the flowing blood is V, the sonic speed in a medium is C, and the angle between the direction of the ultrasonic beam and that of the flowing blood is .theta., the relationship between f1 and f0 is expressed by the following equation (1), and the relationship between fd and V is expressed by the following equation (2). ##EQU1## EQU fd=f1-f0=(2V.multidot.cos .theta./C).multidot.f0 (2)
As is seen from the equation (2), the speed V is in proportion to the deviation frequency fd and in inverse proportion to the angle .theta.. Since, however, vessels complicatedly run in a body, it is actually impossible to measure the angle .theta.. Imaging the flowing blood therefore depends upon the angle .theta. and, in other words, it has an angle dependence characteristic. The angle dependence characteristic causes the following drawbacks.
Even though the speeds of the flowing blood at two observation points (crossings between the flowing blood and the ultrasonic beam) are the same, different results are obtained since the angles 8 at the two observation points differ from each other. It is thus difficult to quantitatively measure the speed of the flowing blood.
Assuming that an observation point in the flowing blood is a sound source, the direction of the flowing blood is divided at the observation point into two directions coming near to and going away from the observation point and, in other words, the positive and negative of the deviation frequency fd are changed to each other at the observation point. The flowing blood is thus displayed in different colors, though the same blood flows in one direction. It is thus necessary to distinguish between arteries and veins and to detect abnormal flowing blood such as shunt and regurgitation in consideration of the angle dependence characteristic, and it is very troublesome to do so.